ON-LINE QUANTITATIVE ANALYSIS METHOD OF THE CONTENT OF

ANTISCALANT COMPOUNDS CONTAINING PHOSPHOROUS IN SEA WATER IN

A REVERSE OSMOSIS DESALINATION PLANT AND CORRESPONDING

CONTROL METHOD AND EQUIPMENT

DESCRIPTION

Field of the invention

The invention relates to an on-line quantitative analysis method of the content of antiscalant compounds containing phosphorous in sea water in a reverse osmosis desalination plant. The invention also relates to a method for controlling a reverse osmosis sea water desalination plant, where the plant has means for adding antiscalant compounds containing phosphorous at a sea water inlet flow and means for regulating and controlling the addition means. The invention also relates to equipment for controlling a reverse osmosis sea water desalination plant.

State of the art

Using the reverse osmosis technique has limitations regarding the quality of the effluyents to be treated. The effectiveness of the reverse osmosis is limited because the membranes become soiled. The pre-treatment of reverse osmosis plants is designed so that the water supplied to the membranes has a low soiling potential, in terms of materials in suspension, particulated material, colloidal and microorganisms. Also it is necessary to foresee the formation of inorganic scale inside the membrane system. Therefore the water supplied has to be pre-treated appropriately. There are various pre-treatment techniques available that use chemical and/or mechanical treatments. The mechanical pre-treatment techniques used are: clarifiers, filters, active carbon filters, sand filters, cartridge filters, etc.

The most common substances in the supply water that soil the membrane are:

- Sediments and particles (clay, colloidal silica, silicates)

- Inorganic scale (calcium carbonate, calcium sulphate, barium and strontium, calcium fluoride, soluble silica)

Metals (iron, manganese, aluminium) - Organic material (coagulants, humic acids, oils and grease)

- . Micro-organisms and material of biological origin

To ensure that no precipitates or scale are formed, a complex analysis must be carried out. This will depend on the chemical composition of the supply water, the conversion percentage ("conversion" is understood to be the percentage of supply water that is converted into permeated water or product), the pH, and finally on the metering, or not, of products that inhibit the formation of scale (also known as antiscalant products). In reverse osmosis, the water concentrated in contact with the membrane is the one which can cause more scale problems. As the water circulates through the membrane, the ions in solution concentrate until they overcome the solubility of certain potentially scale forming salts, causing the formation of precipitates. Reverse osmosis cannot tolerate even the smallest precipitate, even if it is not visible, as it could seriously damage membrane permeability. The pure or permeated water obtained in the separation process cannot cause any problem because it lacks, or virtually lacks, any dissolved salts.

In the beginning when osmosis was used, operating conditions were sought where it was impossible for saline substances to precipitate and, therefore, for scale to form. On one hand, adding high quantities of acid increased the solubility of many potentially scale forming salts. On the other hand, by operating often at relatively low conversions, the saline concentration did not reach critical values.

At present, the implementation of the use of polyamide membranes that do not need acid to be added, because they do not suffer from hydrolysis problems in normal operating conditions, means systems can operate at conditions with a higher scale forming potential. Moreover, with greater water and energy rationalisation, systems operate at the highest conversions possible.

Under these operating conditions, the risk of scale is very high and if competitive plants are to be built, antiscalant additives must be metered accurately into the water supplied to the membrane system. These additives are an essential aid, and the metering thereof must be minimized (optimized), as operating under unnecessarily high concentrations causes unavoidable economic expenses.

Scale inhibitors, or antiscalant agents, used to prevent the formation of scale in the membrane systems are formulae based on molecules that delay the formation of the growth nuclei of the crystals and/or distort the crystalline structure of the scale. The inhibition of crystalline growth is considered to be the most effective method in controlling the formation of CaCO ₃ , CaF ₂ , CaSO ₄ , BaSO ₄ , SrSO ₄ , Ca ₃ (PO ₄ J ₂ , Mg(OH) ₂ , MgCOe scale and silicates in the membrane system.

The control of the metering of antiscalant additives is very important. The lack of antiscalant treatment can render a plant useless, and although many times it is possible to perform a chemical cleaning process whereby it is possible to recover a large part of the permeability that has been lost, the cleaning is complex and time- consuming, and leaves the plant out of service. Therefore it is very important to ensure a certain concentration of inhibitants throughout the time that the reverse osmosis plant is operating, in order to fully protect the membrane system from the precipitation of insoluble compounds. An insufficient dosis, or interrupting the dosage, can result in a serious problem of scale formations on the membrane surface.

One of the usual methods for controlling the metering of the inhibitor is by frequently monitoring the consumption of antiscalant agent. One drawback of this control method is that the concentration of the scale inhibitor is not measured directly, online. Therefore, if there were a fault in the pump, or if the product drum were empty, the appropriate product concentration in the system for preventing the precipitation of the insoluble compounds would not be maintained.

Normally, inhibitor control cannot be performed quickly or in situ, as it involves time- consuming laboratory processes that require long analysis times. On-line inhibitor

- A -

follow-up systems exist wherein an easily measurable fluorescent tracer is used to quantify the level of chemical treatment present in the water supply, the permeated water and in the water concentrate. Nevertheless, using the fluorescent tracer is, at any event, an indirect measurement that can lead to errors because variations in the tracer content do not always correspond to real variations in the inhibitor. Also, it requires an additional compound to be added to the water, which can be significant depending on the subsequent use of the permeated water. No on-line analysis of the active inhibitor material is carried out at any time.

Disclosure of the invention

The aim of the invention is to overcome these drawbacks. This purpose is achieved by means of an analysis method of the type indicated at the beginning and characterized in that it comprises the following stages :

[a] dilution of sea water with dilution water to obtain diluted water, until the diluted water has a conductivity reading of between 25,000 and 3,000 microsiemeπs/cm,

[bj analysis of the concentration of orthophosphate contained in the diluted water,

[c] analysis of the total phosphorous concentration in the diluted water,

[d] determination of the concentration of said antiscalant compounds in the sea water, taking into account the difference between the total phosphorous and the orthophosphate, multiplying by a specific factor of the antiscalant compounds and multiplying by a factor related with the dilution.

In fact, the antiscalant compounds are not detected when analysing the orthophosphates in stage [b] and only appear in stage [c]. Therefore, preferably, stage [c] includes a first part, or substage, wherein the antiscalant compounds containing phosphorous that are present in the diluted water are subject to a chemical treatment in which they become orthophosphates. This way the same

orthophosphate analysing device can be used. Preferably the chemical treatment is hydrolysis.

Through differences, the phosphorous concentration can be determined thanks to s the antiscalant compounds.

Dilution stage [a] is particularly important as high conductivity interferes in the analysis. It must be taken into account that the conductivity values of the sea water vary between 35,000 and 70,000 microsiemens/cm. 0

Preferably, the dilution to be performed depends on the conductivity of the sea water that the inhibitor contains, and which supplies the reverse osmosis plant. The following table shows the dilution factor preferably applied to the dilution tank according to the conductivity of the sea water: 5

Preferably the dilution water has a conductivity reading of less than 1,000 μS/cm, preferably less than 400 μS/cm. In fact, normally a single stage of reverse osmosis is used to desalinate sea water. In other applications, normally two stages are used0 to obtain a better quality (in other words, less conductivity). In the case of reverse osmosis for sea water, the conductivity of the water product or permeated water is usually between 700-100 microsiemens/cm. This permeated water is preferably the one that will be used for diluting, although any other water source could be used. In the case of permeated water resulting from two stages of reverse osmosis, its conductivity reading is usually between 300-10 microsiemens/cm. This water could also be used advantageously as dilution water.

The aπtiscalant agent is metered to the sea water supplied to the reverse osmosis desalination plant, at the beginning. Although there are two stages, the metering is always done in the first stage.

Advantageously, the antiscalant compounds containing phosphorous comprise at least one compound from the group made up of organophosphorated compounds, polyphosphates, phosphinocarboxylic acid and mixtures thereof. Preferably these compounds are also mixed with other compounds, such as for example polycarboxylic acids and/or the salts thereof, wherein it is particularly advantageous that the polycarboxylic acid be a compound from the group made up of homopolymers derived from polyacrylic acid, polymethacrylic acid, polymaleic acid and polyaspartic acid, copolymers and terpolymers of acrylic acid, metacrylic acid, maleic acid, vinylsulphonic acid, alkylsulphonic acid, metalylsulphonic acid, 2- acrylamide-2-metyl-i-propansulphonic acid, vinylphosphonic acid and acryalamide.

Preferably the organophosphorated compounds are organophosphonated compounds, and very preferably they are phosphonic acids and the derived salts thereof (phosphonates).

It is also advantageous that the organophosphonated compounds be compounds from the group made up of phosphonocarboxylic acid, ethylendiaminotetra(methylenphosphonic) acid and the salts thereof, hexamethylendiaminotetra(methylenphosphonic) acid and the salts thereof, diethylentriaminopenta(methylenρhosphonic) acid and the salts thereof, aminotrt(methylenphosphonic) acid and the salts thereof, 1-hydroxylethyliden(1 ,1-diphosphonic) acid and the salts thereof, 2-phosphonobutane-1,2,4-tricarboxylic acid and the salts thereof, morpholinometane diphosphonic acid and the salts thereof, ethanol aminobismethylenphosphonic acid and the salts thereof, ethylentriaminopentakis(methylenphosphonic) acid and the salts thereof, bis(hexamethylen)triaminopenta-(rnethylenphosphonic) acid and the salts thereof,

(2-hydroxyethyl)iminobis-(methylenphosρhonic) acid and the salts thereof.

Preferably the dilution stage is carried out in a conditioning tank and comprises stirring performed with stirring means.

Advantageously the orthophosphate concentration analysis stage is carried out using a colorimetric method.

As already mentioned, preferably the analysis stage of the total phosphorous content comprises a substage wherein the polyphosphates and/or compounds containing organic phosphorous are hydrolysed into orthophosphates, and, advantageously, a substage wherein the orthophosphate analysis is performed using a colorimetric method.

The hydrolysis is preferably carried out using a method from the group made up of digestion with perchloric acid, oxydation with peroxodisulphate, digestion with nitric acid - sulphuric acid and oxydative degradation by ultraviolet radiation.

For its part, the cotormetric method is preferably a method from the group made up of the colorimetric method of vanadomolibdophosphoric acid, the stannous chloride method and the ascorbic acid method.

Generally, both the method for hydrolysing the polyphosphates and/or the compounds containing organic phosphorous into orthophosphates, and the above- mentioned colormetric methods are methods known to a person skilled in the art.

The aim of the invention is also a method for controlling a reverse osmosis sea water desalination plant, where the plant has means for adding antiscalant compounds containing phosphorous at a sea water inlet flow and means for regulating and controlling the addition means, characterized in that an analysis method according to the invention is performed on said salt water inlet flow and the result of said quantitative analysis is sent to said regulation and control means. In fact, the analysis method according to the invention provides obtainable results in a

very short space of time (in a few minutes), which means that a quasi continuous control can be set up, which greatly reduces the risk of scale forming on the membranes since the amount of antiscalant compounds to be added can be modified almost in real time or, where applicable, the desalination process can be interrupted.

Another aim of the invention is a piece of equipment for controlling a reverse osmosis sea water desalination plant, characterized in that it has dilution means and analysis means suitable for performing an analysis method according to the invention.

Brief description of the drawings

Other advantages and characteristics of the invention will be appreciated from the following description, wherein, a preferable embodiment of the invention is described in a non-limiting manner, with reference to the accompanying drawings, wherein:

Fig. 1 is an evolution graph, over time, of the concentration of an antiscalant compound containing phosphorous in a sea water current that supplies a reverse osmosis desalination plant, measured by means of control equipment according to the invention.

Detailed description of some embodiments of the invention

Assay in a pilot reverse osmosis plant :

The antiscalant compound is transported in concentrated format. The concentrated antiscalant compound used is a solution at 25.35% of ethylendiamine tetra(methylenphosphoήic acid) pentasodium salt. The concentrated antiscalant solution is diluted in a dilution tank before being metered continuously to the supply

water. The concentrated antiscalant solution is diluted up to a concentration of 25% with water that has a conductivity reading of 168 microsiemens/cm.

The antiscalant compound diluted in the previous stage is metered continuously at a flow rate of 0.024 millimetres per minute (mL/min) into the water supplied to the plant (which has an approximate flow rate of 4000 mUmin). The concentration of the antiscalant compound in the supply water is approximately 1.5 ppm (parts per million).

The sea water that enters the plant has a conductivity reading of 46,890 microsiemens/cm.

The metered inhibitor concentration in the sea water supplied to the osmosis plant is analysed continuously. The analysis comprises the following stages:

- Stage of diluting the sea water that contains the antiscalant compound. Sea water is pumped from the plant supply effluyent, after metering the antiscalant compound, to the sample conditioning tank. The sea water containing the antiscalant compound is metered into the conditioning tank by a pump at a flow rate of 9.7 mL/min. At the same time, dilution water is metered into the conditioning tank, said dilution water having a conductivity reading of 168 microsiemens/cm, in order to dilute the sea water containing the antiscalant compound. The dilution water is added to the conditioning tank by a metering pump operating at a rate of 29.1 mL/min. The sea water containing the antiscalant compound and the dilution water are mixed in the conditioning tank using a stirrer. The sea water containing the antiscalant compound is diluted at a dilution factor of 4. The conductivity reading of the mixture is 10,930 microsiemens/cm. The time period during which the water remains in the conditioning tank is approximately 60 minutes.

- On-line analysis of the orthophosphate concentration and of the concentration of total phosphorous in the conditioning tank water. The analysis is carried out at 10 minute intervals. A process photometer is used to carry out the analysis to

determine the total phosphorous content. The essential part of the equipment is the unit combining the digestion tub and the photometer. This unit ensures a fast, complete mixture of the sample with the digestion agent and other reagents, fast heating and cooling, and the measurements of the orthophosphate concentration .

1. In a first stage a sample is taken of the conditioning tank water and the orthophosphate content thereof is analysed. To this end, the conditioning tank water sample is fed into the digestion tub using a peristaltic pump. The ascorbic acid method is used to analyse the orthophosphate content. This method is based on the reaction of orthophosphate ions with an acid solution containing molibdate and antimonium ions to form the antimonyl- phosphomolibdate complex. The complex formed is reduced with ascorbic acid to produce a strongly coloured blue molibdene complex. The intensity of the blue colour is proportional to the orthophosphate concentration. The reagents are added to the tub containing the water sample using peristaltic pumps, the tub is shaken and when the reaction finishes the intensity of the blue colouring is measured, taking -into account the zero that has been determined shortly before, and which is considered as a measurement of the phosphate concentration expressed in milligrams per litre of phosphorous (mg/L P).

2. In a second stage a sample is taken of the conditioning tank water and the total phosphorous content is analysed. To this end, the conditioning tank water sample is fed into the digestion tub using a peristaltic pump. In a first stage the polyphosphates and organophosphorated compounds and phosphinocarboxylic acids of the sample are hydrolyzed into orthophosphates via digestion by boiling in a strong acid solution. This is achieved by oxydating the phosphorous compounds with sodium peroxodisulphate in a strong acid solution. The sample is boiled and excess pressure is applied thereto to accelerate the process and this way, short digestion times are achieved. Then the orthophosphate analysis is carried out in the same tub. The ascorbic acid method is used, as explained above. The orthophosphate assessed this way is a

measurement of the concentration of total phosphorous expressed in milligrams per litre of phosphorous (mg/L P).

- Calculations to determine the concentration of antiscalant compound, corresponding in the example to the concentration of the solution at 25.35% ethylendiamine tetra(methylenphosphonic acid) pentasodium salt. The formula used to calculate the antiscalant concentration is as follows:

ConC ⁼ ((Pτotal — > o-phospbate) fd)/f _a πtlsealaπt compound)

Where:

Cone: concentration of the solution at 25.35% of ethylendiamine tetra(methylenphosphonic acid) pentasodium salt in the water supplied to the pilot reverse osmosis plant, expressed in mg/L of antiscalant compound.

Pτo _t a _h concentration of total phosphorous analysed in the supply water, expressed in mg/L of phosphorous.

Po-p _h osp _h a _t «: concentration of orthophosphate analysed in the supply water, expressed in mg/L of phosphorous. fd: dilution of the sea water containing antiscalant compounds, which is performed in the conditioning tank. In the example, dilution 4 was performed.

Fantiscaiant compoun _d : specific constant for each antiscalant compound. In the example, the value of this factor is 0.057516

- The results obtained in the example are shown in Figure 1. The abscissas axis represents the time expressed in minutes, and the ordenate axis represents the antiscalant compound concentration (solution at 25.35% of ethylendiamine tetra(methylenphosphonic acid) pentasodium salt) in the water supplied to the pilot reverse osmosis plant. The average antiscalant concentration during the assay period was 1.549 mg/L. The desired antiscalant concentration in the supply water was 1.5 mg/L. A very exact reading of the antiscalant concentration

was obtained. The standard deviation of the concentrations measured was 1.229 % with respect to the average.

Both in this description and in the claims, it is indicated that the analysis method comprises two stages: the stage of analysing the orthophosphate concentration contained in the diluted water (called stage [b]), and stage of analysing the total phosphorous content contained in the diluted water (called stage [c]). It must be made clear that the method must comprise both stages, but the order thereof is irrelevant. Therefore, it must not be understood that one of the stages must be performed necessarily before the other. Instead, to the contrary, the order in which both stages is performed is completely irrelevant.